Magnetoresistance is the property of a material to change its electrical resistance under the influence of an external magnetic field. The Giant magnetoresistance (GMR) is a type of magnetoresistance that manifests as a significant decrease in electrical resistance in the presence of an applied magnetic field. GMR occurs in thin film structures composed of alternating ferromagnetic and non-magnetic metal layers. The tunnel magnetoresistance effect (TMR) occurs when two ferromagnets are separated by a thin (about 1 nm) insulator, in which case the resistance to a tunneling current changes with the relative orientation of the two magnetic layers. The resistance is normally higher in the anti-parallel case.
In a MRAM cell, a magnetic (hereinafter also “magnetoresistive”) element defined by a thin film structure comprising ferromagnetic materials selected to have magnetoresistance (either GMR or TMR) is used to store data. MRAM devices may have millions of MRAM cells arranged in an array or grid with read and write conductors to enable reading from and writing to the cells.
Each MRAM cell has two stable and distinct configurations that can be selected by rotating a magnetization of the ferromagnetic material used to store data. Each configuration represents a memory state corresponding to a “1” or a “0”.
To write data i.e. a “1” or “0” to a magnetic memory cell, current is passed through read and write conductors in the form of two mutually perpendicular wires which intersect at the memory cell being addressed. MRAM cells respond to the cumulative magnetic field generated by the two mutually perpendicular wires. The magnetic field contributed by each wire is a function of the current density through it.
To realize MRAM devices with greater storage capacity it is desirable to shrink the size of MRAM cells. A consequence of smaller MRAM cells is that the magnetic field required to change the orientation of the ferromagnetic layer on which data is stored is higher. The higher magnetic field required to change the orientation of the ferromagnetic layer may be achieved by reducing the dimensions of the read and write conductors since as noted above the magnetic field generated by these conductors is a function of the current density through the conductors. However, reducing the dimensions of the read and write conductors leads to problems of reliability due to electromigration. Moreover, increases in resistance of the read and write conductors due to their reduced dimensions leads to an increase in the voltage drop. Thus, achievable current densities in the read and write conductors are limited in practice. For the above reasons, efforts to build very high density MRAM devices are being hampered. This is known as the “scaling limitation” hereinafter.